Patent Document 1 and so on are known as examples of attitude stabilization control for a legged mobile robot, in particular a bipedal walking legged mobile robot employing Zero Moment Point (ZMP). Further, Non-Patent Document 1 provides detail with respect to stabilization of the fixed point of a legged mobile body, in particular a passive walker.
Conventional legged mobile robots stand at the forefront of technologies including high-precision sensors, high-performance actuators, and advanced control. The keystone of the technologies is ZMP. A walking control method employing ZMP focuses on trajectory planning and control (including measurement) of ZMP.
Meanwhile, the gait known as passive walking, cannot be ignored, and always generates a great deal of interest in terms of the possibility of engineering applications. A feature of passive dynamic walking is that a natural gait can be produced by the interaction between the dynamics of the legged mobile body and the environment (a downward slope) with no control whatsoever.    Patent Document 1: Japanese Patent No. 3269852    Non-Patent Document 1: T. McGeer, “Passive Dynamic Walking”, The Int. J. of Robotics Research, vol. 9, no. 2, pp. 62-82, 1990